Integrated Services Digital Network (ISDN) is a set of internationally agreed standards specified in the so-called "Blue books" of the International Telegraph and Telephone Consultative Committee (CCITT). ISDN allows voice, data and other applications to communicate over wide bandwidth telephone networks. Essentially ISDN defines a series of layers in a system as well as the interfaces between these layers, through which voice or data can be passed from an application program onto the network and vice versa. Further information about ISDN can be found in e.g. "ISDN Explained" by J Griffiths (Wiley, 1990).
A standard or basic rate ISDN link comprises three channels: one "D-channel" with a data rate of 16 kbits s-1, and two "B-channels", each with a data rate of 64 kbits s-1. The two B-channels are essentially at the user's disposal to send information down, whilst the D-channel is used for signalling--e.g. setting up and finishing calls. Often however signalling does not require the full 16 kbits s-1 capacity of the D-channel, and it is possible to use this spare bandwidth on the D-channel to transmit extra information. CCITT recommendation Q.921 defines a special frame format for use on the D-channel, the Link Access Procedure-D (LAPD). All communication on the D-channel, whether signalling or data transmission, must be in LAPD format.
Another CCITT digital communications standard is X.25. This has its own frame format, known as LAPB ("Link Access Procedure--Balanced"), as well as a physically different socket connection from ISDN. X.25 transmission lines typically have a bandwidth of 9.6 kbits s-1. Recommendation X.31 of the Blue Books discusses the possibility of transmitting signals from an X.25 terminal into an ISDN link, allowing both X.25 and ISDN systems to be used whilst only maintaining a single type of connection (see also "ISDN explained", p107). Recommendation X.31 proposes a terminal adapter which intercepts frames from an X.25 system, and converts them into ISDN format for transmission over an ISDN link. A second terminal adapter could be used if necessary to convert the received frames back into X.25 format (depending on the nature of the telephone network being used), with the ISDN part of the link being transparent to the X.25 systems.
It is desirable to be able to transmit the X.25 frames over both the B-channel and the D-channel. The former is relatively simple, because the network allows any format to be used on the B-channel, but there is a problem with the latter in that only LAPD frames may be used on the D-channel, whereas the X.25 frames are in LAPB format. The terminal adapter therefore needs to be able to convert LAPB frames into LAPD, and vice versa at the receiving end. Recommendation X.31 suggests that this conversion can be performed by full link layer termination, but this requires a considerable amount of processing and is therefore relatively slow (the conversion is performed not in the link layer but in the packet layer instead, so that two logical links must be supported). Recommendation X.31 (section 7.4.2.2) also mentions the possibility of directly mapping from LAPB into LAPD but does not describe how this might be achieved.